1. Field of the Invention
This invention relates to lasers, and more particularly it relates to a transverse discharge excitation arrangement for waveguide gas lasers.
2. Description of the Prior Art Including Prior Art Statement
Recently there has been considerable interest in waveguide gas lasers wherein the laser light propagates through a hollow waveguide which also serves to confine the laser-exciting discharge. Early forms of waveguide gas lasers are disclosed in U.S. Pat. No. 3,772,611, issued Nov. 13, 1973 to Peter W. Smith. The basic laser excitation scheme disclosed in this patent and used in most of the early waveguide gas lasers involves establishing a dc electric discharge longitudinally through the device between a pair of electrodes disposed near the respective ends of laser waveguide. This type of discharge requires relatively large dc excitation voltages of around 10 kv along with the necessary power supply and associated circuitry for generating voltages of this magnitude.
The aforementioned Smith patent also briefly discloses exciting a ring-type waveguide laser from an rf source by means of a coil wound about the ring-shaped waveguide. Such a coil-type excitation arrangement not only is incapable of providing a highly uniform discharge, but it also affords poor coupling efficiency. Moreover, when more than a few coil turns are employed, the inductance of the coil becomes sufficiently large to limit the usable excitation frequencies to below a few MHz.
In order to obtain a more uniform discharge with reduced excitation voltage, waveguide gas lasers have been developed wherein a pulsed discharge is established along a transverse waveguide dimension. Transversely excited waveguide lasers are disclosed in U.S. Pat. No. 3,815,047 issued June 4, 1974 to Smith et al. Waveguide lasers of the type described in the Smith et al patent have been operated in a quasi-continuous mode at pulse repetition rates as high as 40 kHz, as described in the Smith et al paper "High Repetition-Rate and Quasi-CW Operation of a Waveguide CO.sub.2 TE Laser", Optics Communications, Vol. 16, No. 1 (January 1976), pp. 50-53.
In both of the aforementioned longitudinal and transverse electric discharges, the cathodes usually are sufficiently poor electron emitters so that positive ion current dominates in the region immediately adjacent to the cathode, and as a result, a positive space charge is formed in this region. The electric fields resulting from this positive space charge cause electrons emitted from the cathode to be accelerated sufficiently so that an avalanche ionization effect occurs in the space charge region. By the outer extremity of the space charge region the electron density is sufficiently large so that an electron dominated current occurs throughout the remainder of the discharge. Since in the space charge region the discharge voltage increases very rapidly in a positive direction as a function of distance from the cathode (typically by about 400 to 600 volts in waveguide laser-exciting transverse discharges), the space charge region is often referred to as the "cathode fall" region. Throughout the remainder of the discharge, i.e., between the cathode fall region and the anode, the discharge voltage increases very slowly in a positive direction as a function of distance from the cathode.
The aforementioned and other effects in the cathode fall region give rise to a number of problems in previous discharge-excited waveguide lasers. First, positive ion bombardment of the cathode has a tendency to damage the cathode, thereby limiting the life of the device. Also, the high electric fields in the cathode fall region tend to dissociate the laser gas. In addition, the relatively large cathode fall voltage wastes a substantial amount of input energy, thereby reducing operating efficiency. Further, considerable additional circuit hardware such as high voltage power supplies, current regulators, and ballast resistors may be required to provide the relatively large excitation voltages as well as to overcome instabilities resulting from negative impedance discharges. Moreover, in pulsed transverse discharge excitation of the prior art, the excitation pulse duration must be sufficiently short to preclude arcing, and bulky and expensive pulse-forming networks are required.